This invention relates to the casting of steel strip by a single or a twin roll caster. In a twin roll caster, molten metal is cast into strip through a pair of counter-rotated horizontally positioned casting rolls, which are internally cooled so that metal shells solidify on the moving roll surfaces, and are brought together at the nip between them to produce a thin cast strip delivered downwardly from the nip. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel, such as a tundish, from which it may flow to a distributor and then through a metal delivery nozzle located above the nip forming a casting pool of molten metal supported on the casting surfaces of the rolls. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the casting rolls so as to dam the two ends of the casting pool against outflow.
When casting steel strip in a twin roll caster, the casting pool will generally be at a temperature in excess of 1550° C., and usually 1600° C. and greater. It is necessary to achieve very rapid cooling of the molten steel over the casting surfaces of the rolls in order to form solidified shells in the short period of exposure on the casting surfaces to the molten steel casting pool during each revolution of the casting rolls. Moreover, it is important to achieve even solidification so as to avoid distortion of the solidifying shells that come together at the nip to form the steel strip. Distortion of the shells can lead to surface defects known as “crocodile skin surface roughness”. Crocodile skin surface roughness is known to occur with high carbon levels above 0.065%, and even with carbon levels below 0.065% by weight carbon. Crocodile skin roughness, as illustrated in FIG. 1, is known to occur for other reasons. Crocodile skin roughness involves periodic rises and falls in the strip surface of 40 to 80 microns, in periods of 5 to 10 millimeters, measured by profilometer.
We have found that with carbon levels below 0.065% by weight the formation of crocodile skin surface roughness is directly related to the heat flux between the molten metal and the surface of the casting rolls, and that the formation of crocodile skin roughness can be controlled by controlling the heat flux between the molten metal and the surface of the casting rolls. FIG. 2 reports dip tests that illustrate the relationship between the heat flux and the formation of crocodile skin roughness during the formation of the metal shells on the surfaces of the casting rolls in making the thin cast strip. As shown by FIG. 2, we have also found that by controlling the energy exerted by rotating brushes peripherally in contact with the casting surfaces of each casting roll, in advance of contact of the casting surface with the molten metal, that the heat flux between the molten metal and the surface of the casting rolls, and in turn crocodile skin surface roughness on the resulting thin cast strip, can be controlled.
This relationship between the heat flux from the molten metal and the surface of the casting rolls and the formation of crocodile skin surface roughness on the thin cast strip has been found to occur whether the casting roll surfaces are smooth or textured. FIG. 3 reports dip tests that illustrate how the heat flux is changed with both smooth and textured casting surfaces on the casting rolls. We have also found that the texture of the casting roll surfaces of the casting rolls change during casting. This change can cause a change in heat flux from the molten metal to the casting roll surfaces and in turn a change in formation of crocodile skin surface roughness on the thin cast strip. We have found a method of directly controlling the formation of crocodile skin surface roughness by controlling the heat flux between the molten metal and the casting roll surfaces, to avoid high fluctuations in the heat flux during the formation of the metal shells during casting and in turn control the forming of crocodile skin surface roughness in the thin cast strip produced.
A method of controlling the formation of crocodile skin surface roughness comprises the steps of:                directing an electromagnetic beam source toward the surface of thin cast strip following discharge from casting surfaces of a twin roll caster;        detecting reflectance of the electromagnetic beam source from the surface of the thin cast strip;        processing the detected reflectance from the surface of the thin cast strip to measure the degree of roughness of the surface of the thin cast strip; and        based on the measured degree of roughness, controlling the degree of cleaning of the casting surfaces by controlling energy exerted by brushes against the casting surfaces of the twin roll caster to control crocodile skin roughness of the thin cast strip.        
Alternately, the method of controlling the formation of crocodile skin surface roughness in continuous casting of thin cast strip is disclosed that comprises the steps of:                assembling a pair of counter-rotating casting rolls laterally to form a nip between circumferential casting surfaces of the rolls through which metal strip may be cast;        forming a casting pool of molten metal of carbon steel of less than 0.065% by weight carbon supported on the casting surfaces of the casting rolls above the nip;        assembling a rotating brush peripherally to contact the casting surface of each casting roll in advance of contact of the casting surfaces with the molten metal in the casting pool;        directing at least one electromagnetic beam source toward at least one of the casting roll surfaces;        detecting the reflectance of at least one electromagnetic beam source from the casting roll surface directed to the surface from the electromagnetic beam source and generating an electronic signal corresponding to the detected reflectance from the casting surface;        monitoring the degree of cleaning of the casting surfaces of the casting rolls based on the detected reflectance of the electromagnetic beam source from the casting surface of the casting rolls;        controlling the energy exerted by the rotating brushes against the casting surfaces of the casting rolls based on the monitored degree of cleaning to expose a majority of projections of the casting surfaces of the casting rolls and provide wetting contact between the casting surface and the molten metal of the casting pool; and        counter-rotating the casting rolls such that the casting surfaces of the casting rolls each travel toward the nip to produce a cast strip downwardly from the nip.        
The electromagnetic beam source may be directed to contact the casting roll surface after contact with the rotating brush and before entry into the casting area where a controlled atmosphere is maintained above the casting pool. The electromagnetic beam source may be directed to contact the casting roll surface adjacent the rotating brush.
The methods may include detecting the specular reflectance, detecting the diffuse reflectance, or both. A signal may be provided to a device selected from the group consisting of a voltmeter, chart recorder and data logger.
The energy of the rotating brush against the casting roll may be controlled by varying the pressure applied by the brush against the casting roll surface of the casting roll, varying the rotation speed of the brush against the casting surface of the casting roll, or by both the applied pressure and the rotation speed. The energy, applied pressure and rotation speed of the rotating brush against the casting roll may be measured by measuring the torque of a motor rotating the brush. The energy may be automatically controlled by automated controls during a casting campaign.
By controlling the degree of cleaning based on reflectance of the roll surface, the same effective cleaning of the casting surfaces can thus be controlled and maintained through the casting campaign. In turn, the cleaning of the casting surfaces can be monitored and controlled indirectly by controlling the energy exerted by the rotating brush against the casting rolls either manually or automatically as explained in detail by example below.